I came accross something on the internet about a 'dry storm' over the
weekend. but now I can't find it.  From what I remember a dry storm is
a rain storm but the rain evaporates before it hits the ground because
of a dry layer of air between the rain and the earth (lightening often
hits and starts fires because the ground is dry).  I found this
fascinating and would like more info.
Here is what I want:
1. Information on a dry storm including the conditions necessary for
this to occurr. What type of air masses or fronts need to be present.
I would prefer 3 or 4 short paragraphs summarizing the info found on
dry storms.
2. One or two diagrams showing how this weather condition develops.

Hello crabapple and thank you for your question.

The 'dry storm' weather system that you describe is also known as 'dry
lightning'  This type of weather system is associated with storms that
are high in the atmosphere (upto 18,000 feet) and are associated with
air content that is low in humidity situated slightly lower than the
actual storm.  As you note in your question, because of the high
altitude and low humidity, little or no rain actually hits the ground.
 These high altitude storms often cause gusty conditions at ground
level.

The sites I have researched below will give you a more detailed
description as to the exact process, and several of them contain
diagrams as requested.


Note:*Link in PDF format*
"Dry Lightning Storm: Thunderstorm in which negligible precipitation
reaches the ground. Also called a dry storm"
http://www.fire.blm.gov/textdocuments/glossary.pdf

An interesting academic article including maps, graphs etc..
Note:*Link in PDF format*
"Fuquay et al. (1979) described high-level dry thunder-storms in the
northern Rocky Mountains that usually are triggered by an upper-level
cold front or other cold air advection aloft. Little or no
precipitation from these storms reaches the ground from their bases,
which can be as high as 3000 m above ground level"
http://www.fs.fed.us/pnw/fera/publications/fulltext/amslightning.pdf

A good paper albeit a little old (1976 but still valid) with some
interesting diagrams: Note:*Link in PDF format*
"High level thunderstorms, with bases between 12 and 18 thousand
feet,are fairly common over the Western Region from May through
September.Experience has shown that these storms are often associated
with strong gusty surface winds and "dry lightning". While most
thunderstorms are noteworthy for having heavy rains, this variety
often has little or no precipitation reaching the ground. They are
identifiable by their high bases, and by the appearance of virga.
Viewed from a distance, a heav yrain curtain from the cloud base
(giving an opaque bluish or greyish appearance) will often evaporate
out to a thin curtain (which can beseen through) near the surface.
Below the cloud base the air is originally quite dry. This results in
considerable evaporation of the rain produced by the cumulonimbus
clouds (CBs)."
http://205.156.54.206/pub/im/wrta7614.pdf

"There are so many fires out west this year and we keep hearing that a
lot of them are being started by "dry lightning." Could you please
explain to those of us that live in a wet Great Lakes area what
exactly "dry lightning" is and what the difference is to the lightning
that we experience in a thunder storm with rain?

Well, that's precisely the difference. A thunderstorm with rain
produces "wet" lightning I guess you could call it. A thunderstorm
without rain produces dry lightning. A thunderstorm without rain
actually does have rain; it's just that the rain dries up before it
reaches the ground. This type of storm is common in the west where the
air is considerably drier than it is generally east of the
Mississippi.

Because it's so dry in the west, thunderstorms form much higher in the
sky, so high that when the rain falls it often evaporates before
reaching the ground. But the lightning generated in the storm, the
same way it does in any storm east or west, CAN reach the dry land
below, even if the base of storm itself is two, three or more miles
high. When lightning does strike, a fire is sparked but there's no
rain falling to douse the flames. And this summer the unremitting heat
dried the forests to the point were any spark, especially from dry
lightning, could turn to a blaze in seconds."
http://www.weathernotebook.org/transcripts/2000/10/30.html

A rather basic diagram of a 'dry thunderstorm'
http://www.usatoday.com/weather/wdryzap.htm

LIGHTNING ACTIVITY LEVELS (LAL)from 1 - 6
"LAL 6 - DRY LIGHTNING describes this rare but very significant event.
In such situations cloud bases tend to be high - generally above
15,000 ft MSL, with no significant wetting precipitation. In most
high-based thunderstorms the frequency of cloud-to-ground lightning
strikes is low...yet a very high number of these strikes start fires
because the sub-cloud base environment is very dry and fuel moistures
in the smaller fuels is very low. Some of the larger lightning-caused
fires are those undetected by IR areal reconnaissance lights.

Whenever the potential exists for dry lightning, the forecaster should
issue a RED FLAG WARNING or a FIRE WEATHER WATCH. In several areas of
the western states, the first day of a thunderstorm outbreak (after an
extended dry period), has the highest potential for new lightning
caused wildfires. As the respective pattern progresses, more moisture
is normally advected into the region - reducing the risk of LAL 6.
However, in areas of the western mountain states, extended periods of
dry lightning activity does occur. Keep in mind that in the western
mountainous states, dry lightning only occurs on approximately 1-2% of
all lightning days."
http://www.seawfo.noaa.gov/fire/LAL.htm

A fairly lengthy and scientific proposal.
"We propose to incorporate existing weather predictions into tactical
fire preparedness and planning by adapting a methodology to assess the
risk of "dry" lightning (that which occurs without accompanying
rainfall). Based on atmospheric moisture and stability variables, we
will develop discriminant rules that assign a probability of dry
lightning over the United States, using a method that we created and
tested in the northwestern U.S. (Rorig and Ferguson 1999)."
http://www.fs.fed.us/pnw/fera/jfsp/lightning/lightningbriefing.html

Thank you for the question, and if you need any clarification of my
answer do not hesitate to ask.

Very best regards

THX1138

Search strategy included:
"dry thunder"  lightning -robe -rope
://www.google.com/search?q=%22dry+thunder%22++lightning+-robe+-rope&hl=en&lr=&ie=UTF-8&as_qdr=all&start=0&sa=N

"dry lightning" 
://www.google.com/search?hl=en&lr=&ie=ISO-8859-1&as_qdr=all&q=%22dry+lightning%22+&btnG=Google+Search

Also you can keep up to date on information regarding Dry Lightning by
using Google News:
"dry lightning" 
http://news.google.com/news?q=%22dry+lightning%22+&hl=en&lr=&ie=UTF-8&as_qdr=all&sa=N&tab=wn